Photograph of a hard-start capacitor suitable for an air conditioner compressor motor or heat pump compressorCauses of Hard-Starting Electric Motors
Hard Starting Air Conditioning / Heat Pump Compressor Motor & Other Electric Motors      

  • CAUSES of HARD STARTING ELECTRIC MOTORS - CONTENTS: what causes an electric motor to have trouble starting-up? Understanding why a motor is not starting leads to the proper repair. Replacing bad start capacitor may be a simple and permanent repair or it may simply postpone the need to replace a failing electric motor or compressor.
  • POST a QUESTION or READ FAQs about installing a hard-start capacitor to get an air conditioner motor, fan motor, or other electric motor running.

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Hard-starting electric motor diagnosis & repair:

What causes hard-starting compressor motors, fan motors, or other electric motors?

Understanding why an electric motor is having trouble starting or why it won't keep running is an important diagnostic step in keeping an electric motor or air conditioner, heat pump, well pump or other motor working properly. Understanding why a motor is hard starting also tells us if it makes sense to install a hard-start capacitor on the equipment.

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Causes of Hard-Starting Air Conditioners, Refrigerators, Freezers, and other Electric Motors

A compressor or other motor may have trouble starting for any of a variety of reasons such as:

Electric motor starting capacitor (C) Daniel Friedman
  • Low line voltage supplied to the equipment or excessive power drop on a long circuit wire
  • Un-balanced cooling or refrigeration systems - such as a compressor motor that is having trouble starting when the refrigerant pressures are high on one side and low on the other side of the cooling system. This hard starting condition happens when a compressor is turned off in the middle of an "on" run cycle.

    You can guess that you have this condition on an air conditioner if the system starts just fine when it's been shut down for a half hour or more (refrigerant pressures are equalized on both sides of the compressor) but starts with difficulty (the compressor motor "hums" and does not start quickly or may even trip the circuit breaker) when trying to restart a few minutes after having been shut off.
  • Old, aging compressor motors or other electrical motors that are at or near the end of their life may have trouble starting and may be able to function for some additional time given the "help" provided by a starting capacitor.


    for more details about old or failed compressor motors.
  • Troubles with an electric motor such as used on an air conditioner air handler, condensing coil fan, or a well water pump:

  • Bad or failed starting capacitor: the air conditioner compressor (and some other electric motors) may already have a starting capacitor installed, but the starting capacitor might have failed, causing the air conditioner compressor to start with difficulty or not at all. A bad starter capacitor can also disable the fan in the outdoor compressor/condenser, or the blower fan in the indoor air handler unit.

    Capacitors are electric devices that get an electric motor running at start-up or that help keep a motor running once it has started. If the capacitor has failed the symptom is that the motor won't start. You may hear it humming or observe that it's getting hot. If you observe this we suggest that to avoid damage you turn off the system while waiting for repairs.

    We give more detail about failed electric motor starting capacitors, and we explain possible visual diagnosis of a failed starter capacitor (bulged ends) without having to perform electrical testing,
    Thanks to reader George Fazio for this air conditioner start-up diagnosis suggestion and for the photo (above left).

Starting capacitors or electric motor starting capacitors (or motor start boosters) are often present on large single phase air conditioning compressors, as found on home air conditioning units, or on occasion on blower motors or even fan motors.

Electric motor starting capacitors are only very rarely present on small refrigeration compressors, such as those in refrigerators, and as far as we know, never present on 3-phase power systems.

  • Bad centrifugal switch in the electric motor: Thanks to a reader, Paul J. Ste. Marie we can add that a bad centrifugal switch can also keep an electric motor from starting or running as it should.

    A centrifugal switch as used in some electric motors uses the centrifugal or rotating force of the motor to open or close an electrical switch. Electrical current applied to a start winding causes an electrical field placed at an angle to the field produced by the main or "run" winding to provide an initial rotating force to start the motor.

    In these split-phase electric motors a centrifugal switch is used to switch off electrical power to the start winding once the motor has gotten up to a minimum speed of about 75% of its full run velocity. In essence the centrifugal switch turns off the start circuit and leaves the motor operating from its run-circuit alone.

    This application of centrifugal switches is common in split-phase or "induction-start / induction-run alternating current (AC) electric motors that do not require a high starting torque, such as small power tools, grinders, etc. - citation. - Heinecke, Kevin (2015) at REFERENCES.

    Ste. Marie pointed out that

Another common reason for a CS or CSCR motor to fail to start is a bad centrifugal switch. If the switch contacts are dirty, the motor can fail to start. If the switch freezes closed, the start capacitor will overheat. - P.S.M. 10 Jan 2015

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Continue reading at HOW a STARTING CAPACITOR WORKS or select a topic from the More Reading links shown below.

or see ELECTRIC MOTOR DIAGNOSTIC GUIDE for help in testing a bad-behaving electric motor or an electric motor that won't start

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